MCP6141 datasheet(13/34 Pages) MICROCHIP

13 / 34 page

DS21668B-page 13

MCP6141/2/3/4

EQUATION 4-2:

In order for the amplifiers to be stable, the noise gain

should meet the specified minimum noise gain. Note

that a noise gain of GN = +10 V/V corresponds to a

non-inverting signal gain of G = +10 V/V, or to an

inverting signal gain of G = -9 V/V.

FIGURE 4-2:

Noise Gain for Non-inverting

Gain Configuration.

FIGURE 4-3:

Noise Gain for Inverting

Gain Configuration.

Figure 4-4 shows a unity gain buffer and Miller integra-

tor

that

are

unstable

when

used

with

the

MCP6141/2/3/4 family. Note that the capacitor makes

the

integrator

circuit

reach

unity

gain

high

frequencies, which makes these op amps unstable.

FIGURE 4-4:

Typical Unstable Circuits for

the MCP6141/2/3/4 Family.

4.4.2

CAPACITIVE LOADS

Driving large capacitive loads can cause stability

problems for voltage feedback op amps. As the load

capacitance increases, the feedback loop’s phase

margin decreases and the closed-loop bandwidth is

reduced. This produces gain peaking in the frequency

response, with overshoot and ringing in the step

response. A unity gain buffer (G = +1) is the most

sensitive to capacitive loads, though all gains show the

same general behavior.

When driving large capacitive loads with these op

amps (e.g., > 60 pF when G = +10), a small series

resistor at the output (RISO in Figure 4-5) improves the

feedback loop’s phase margin (stability) by making the

output load resistive at higher frequencies. The band-

width will be generally lower than the bandwidth with no

capacitive load.

FIGURE 4-5:

Output Resistor, RISO

stabilizes large capacitive loads.

Figure 4-6 gives recommended RISO values for differ-

ent capacitive loads and gains. The x-axis is the nor-

malized load capacitance (CL/GN), where GN is the

circuit’s noise gain. For non-inverting gains, GN and the

Signal Gain are equal. For inverting gains, GN is

1+|Signal Gain| (e.g., -9 V/V gives GN = +10 V/V).

FIGURE 4-6:

Recommended RISO Values

for Capacitive Loads.

After selecting RISO for your circuit, double check the

resulting

frequency

response

peaking

and

step

response overshoot. Modify RISO’s value until the

response is reasonable. Bench evaluation and simula-

tions with the MCP6141/2/3/4 SPICE macro model are

helpful.

-------

10 V/V

≥

VIN

MCP614X

RIN

VOUT

MCP614X

RIN

VOUT

VIN

MCP614X

VOUT

VIN

MCP614X

VOUT

VIN

Unity Gain Buffer

Miller Integrator

MCP614X

RISO

VOUT

1,000

10,000

100,000

1.E+00

1.E+01

1.E+02

1.E+03

Normalized Load Capacitance; CL/GN (F)

100k

10p

GN = +10

GN = +20

GN t

+50

10k

100p

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